This research seeks new knowledge of biophysical mechanisms by which spontaneity of a neuron gives rise to rhythmic patterned activity. The isolated cardiac pacemaker of crustaceans permits separation of the role of endogenous neuronal characteristics from that of neuronal interaction, in integrative performance by the system. The identification of spontaneous, rhythmic depolarizations recorded intracellularly after impulse activity is silenced by procaine or tetrodotoxin (TTX) treatment as 'driver potentials' will be tested by comparing responses to depolarizing current pulses with neuronal activity pattern changes to equivalent stimuli in fully functioning ganglia. Membrane permeability changes involved in generation of the TTX resistant potentials will be assessed by observing changes in membrane polarization in response to small, constant current pulses, and effects of salines of altered ionic composition. Effects of potassium removal, ouabain, cooling, and other procedures known to alter active sodium transport will be sought. The mode of action of a neurohormone having specific cardioexcitor activity will be tested on the TTX resistant potentials. Separation of the contribution of endogenous neuronal characteristics from the contribution of neuronal interactions to the integrated output of the ganglion will be undertaken. Previously developed techniques for simultaneous unit analysis from multichannel extracellular recording, localized application of substances, and intracellular current passing will be applied. The effect of shutting off a single cell, or selectively interrupting impulse mediated interactions, and of otherwise perturbing the activity of one or a few units will be studied to detail the integrative mechanisms ensuring the remarkable reliability of the pacemaker function of this ganglion and its independence from individual cell activity despite small cell numbers.